Laser photofragment spectroscopy of jet-cooled CClF2NO

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CHEMICAL PHYSICS LETTERS

8 May 1987

LASER PHOTOFRAGMENT SPECTROSCOPY OF JET-COOLED CCIFzNO M.R.S. McCOUSTRA, J.A. DYET and J. PFAB Department of Chemutry, Heriot- Watt University, Edinburgh EH14 4AS. UK

Received 26 January 1987; in final form 5 March 1987

Photofragment yield spectra of jet-cooled CClF,NO in the visible region have been recorded using pulsed laser dissociation in conjunction with two-photon LIF probing of the nascent NO fragment. Photodissociation of the cold parent at 646.6 nm yields NO with a statistical rotational state distribution but a colder than statistical spin-orbit distribution. A dissociation energy of 13500 f 350 cm-’ has been evaluated for the C-N bond of the parent nitroso compound.

1. Introduction

2. Experimental

Among the array of sophisticated experimental methods at the disposal of gas-phase photochemists dissociation-probe techniques utilising pulsed tunable lasers are now beginning to occupy a prominent place [ I]. Photodissociation experiments achieving a high degree of state selection in the excitation step are particularly informative and can yield state-specific dissociation rates, nascent product state distributions and information on potential energy surfaces [ 21. The C-nitroso compounds are very promising candidates for studying the predissociation dynamics of larger polyatomic molecules in this way as has been shown by recent studies on CF3N0 [ 3 1, NCNO [ 4-61 and 2-methyl-2nitrosopropane [ 71. Here we report preliminary results from a detailed study of the predissociation dynamics of jet-cooled CCIFzNO using state-selective excitation in the red and delayed two-photon laser-induced fluorescence (LIF) for probing nascent nitric oxide. As is the case with CF,NO whose predissociation has been examined carefully in this way [ 31, the A+% n,x* transition in the 550 to 750 nm range is structured but too congested to permit significant vibronic state selection at 300 K. The x($x*) state of CCIFINO also emits fluorescence, and both excitation and dispersed fluorescence spectra of the jet-cooled molecule in the visible and near-IR have been recorded and partially analysed [ 8 1. The electronic origin of the A+% system has been located at 704.7 nm [ 81.

CClF*NO was prepared from tetrafluoroethylene by a two-step synthesis [ 91 and purified by low temperature fractionation under vacuum. The pulsed jet apparatus consisted of a small cubic chamber constructed of aluminium with short baffle arms and quartz windows for admission of the laser beams. A pulsed solenoid valve with 0.5 mm orifice was mounted on the top face of the chamber opposite the evacuation port. A 600 II s- ’ oil diffusion pump maintained a background vacuum of about 1Op4Torr during routine operation of the nozzle at a repetition rate of 10 Hz. 5 to 10% mixtures of CCIFzNO in Ar and stagnation pressures of 400 Torr were used in combination with an opening time for the pulsed valve of 150 ps. Under these expansion conditions two-photon LIF spectra of NO indicate that the rotational temperature is less than 10 K. Fluorescence in the UV was collected in a direction normal to the laser beams and jet axis by f/l fused quartz optics and detected by a solar blind photomultiplier. The detector signals were averaged either by a gated integrator/boxcar averager or by computer in conjunction with a transient recorder. A JK 2000 Nd-YAG pumped dye laser provided tunable red pulses for dissociation (15 ns, 0.3 to 3 mJ, 0.2 cm -’ band width), and an excimer pumped dye laser (Lambda Physik EMG 50E/F12002) was employed for delayed probing of NO by two-photon LIF near 450 nm at pulse energies near 0.5 mJ. The

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two focused beams were aligned collinearly and crossed the free jet some 5 to 10 mm downstream of the orifice. The temporal delays between the opening of the valve and the dissociation and probe laser pulses were adjusted by a multiple analogue delay generator in conjunction with a crystal-controlled master oscillator. The combined jitter in the delay between the laser pulses was roughly 20 ns.

3. Results The NO fragment Was probed by two-photon LIF from the v’ = 0 level of the A 2X+ state. These transitions of the y-band system have favourable Franck-Condon factors, the two-photon excitation spectra exhibit resolvable rotational structure [ lo], and the technique has been used with success in previous studies of the photodissociation of NO-containing molecules [3,4,1 l-141. Variation of the delay between the dissociation and probe pulses provided rough appearance and disap-

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pearance rates of NO when the probe laser was tuned to a strong feature in the excitation spectrum of the fragment. The initial rise in the number density of NO is followed by a maximum and then a slower decrease that largely reflects the rate at which nascent NO moves out of the focal volume of the probe laser. The crude rise times evaluated from such plots of relative NO number density versus delay time range from 250 ns at the origin of the n,n* system near 14 192 cm- ’ to less than 20 ns when parent features are excited above 15400 cm-’ by the dissociation laser. These measurements will be reported in detail elsewhere. 3.1. Internal energydistributionsof NO Fig. 1 shows a section of the two-photon LIF spectrum of nascent NO with the photolysis laser tuned to the strong absorption feature of the parent at 646.6 nm. An NO rise time measurement with the probe laser tuned to the intense Ozl + PI, band head showed that the rate of dissociation at this wavelength exceeds

4%

453 h/nm

Fig. 1. Section of two-photon LIF spectrum of nascent NO(X#

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=0) from the 646.6 nm photolysis of jet-cooled CClF,NO.

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Table 1 646.6 nm photodissociation of CClF,NO: C-N bond dissociation energy 08, average available energy of the jet-cooled parent ( Eayl) , average rotational, vibrational and electronic energies, observed and calculated (in parentheses) statistical spin-orbit population ratios of the nascent NO D~=13500+350cm-’ (&,,)=1970+350cm-’ (&,,)=393*40cm-‘(20%) (Evlb) =4 cm-‘(0.2%) (&,)=31+5cm-‘(1.6%) F,/F,= 3.03(calculated I .35)

the 20 ns duration of the dissociation pulse. A fixed delay of 100 ns was employed for scanning the fragment LIF spectrum shown in fig. 1. The spectrum is easily assigned by comparison with thermalised NO at room temperature showing that the low .Zlevels are populated preferentially. The inversion of the spectrum to obtain the rotationally resolved level populations n(J”) of the NO after correction of the observed relative intensities Z,, for several experimental variables was accomplished by means of the following relationship:

8 May 1987

by identifying the highest observable vibration -rotation quantum state for which the translational energy is zero in the molecular frame. The signal-tonoise ratio achieved in this experiment did not permit the use of this procedure. A less accurate indirect procedure based on the comparison of calculated statistical rotational distributions [ 161 of NO with the observed population distribution as outlined by Bower et al. [3] had to be employed instead and resulted in a value of 13500+ 350 cm- * or 162 kJ mol-‘. 3.3. NO fragment yield spectra By tuning the dissociation laser through the vibronic features of the n,x* band of the nozzlecooled parent while keeping the probe laser fixed to thestrong02,+P,, headofNO(*II,,*) onthey(O,O) band at 452.6 nm we obtained NO yield spectra with good signal-to-noise ratio. The example shown in fig. 2 was recorded with a delay of 75 ns between dissociation and probe pulses. Such spectra reflect the yield of NO for specified 8, v and .Zand represent the convolution of the parent absorption cross section with

where Dis the frequency in cm-’ and SJ9,.. is the twophoton line strength of a S cs’ transition [ 151. Some spectral lines with obviously too low intensity were considered to be affected by multiple photon resonances and were not included in the analysis. The rotational distributions obtained in this way for both spin-orbit states of NO (F, - *II ,,*, F2 - *II 3l2)were used to evaluate the average rotational energy and spin-orbit population ratio. NO( v= 1) could not be observed, and signal-to-noise considerations indicate that less than 1 x IO-* ofthe NO could be vibrationally excited. Table 1 summarises these results including values for the available excess energy and the C-N bond dissociation energy. 3.2. C-N bond dissociationenergy Provided expansion conditions permit adequate cooling of the internal degrees of freedom of the parent dissociation probe, experiments in nozzle-cooled jets can provide accurate bond dissociation energies

Fig. 2. NO fragment yield spectrum obtained by probing nascent NO on the 02, +P,, bandhead of the y (0,O) band at 452.6 nm using a delay of 75 ns between dissociation and probe pulses. The spectrum has not been corrected for variations in dye laser power with wavelength.

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the quantum yield for dissociation. The fragment yield spectrum of fig. 2 complements the fluorescence excitation spectrum [ 81. Similar scans to shorter wavelengths extend the spectroscopic range to high-lying vibrational levels of the upper state that cannot be seen in emission due to rapid non-radiative relaxation followed by dissociation. Our spectra confirm the location of the 0,O origin and also show the weak satellites noted previously in the excitation spectrum [ 8 1. We consider that these puzzling weak features should to be attributed to “hot” transitions. A detailed study of the A+% system based on the initial work [ 81 and including torsional level simulations and rotational contour analysis as well as fluorescence decay measurements will be reported elsewhere [ 171.

4. Discussion The structured electronic spectrum of CCIFzNO in the 550 to 720 nm region, the radiative properties of the upper electronic state involved in this transition and the appearance times of the nitric oxide fragment all indicate that the photodissociation of CC1F2N0 in the visible region takes place on the nanosecond time scale and can be safely classified as a typical predissociation. The rotational population distribution of each spin-orbit state of the nascent NO produced by photolysis on the 646.6 nm vibronic transition is statistical but an FJF, branching ratio of less than half the expected statistical ratio is observed. Distinct preferences for the lower spin-orbit component ( F1) have been found previously in the photodissociation of C-nitroso compounds [ 3,7,14]. The observation of such colder than statistical electronic branching ratios may not necessarily be the result of dissociation from a triplet surface, although this is most likely in the present case. We have chosen to present here our results obtained by excitation of the 646.6 nm feature, although it forms part of the congested region of the spectrum 1275 cm- ’ above the 0: origin and about 2000 cm-’ above the dissociation threshold where a confident vibronic assignment is no longer possible. Our results using excitation of assigned vibronic features closer to the dissociation threshold can be summarised 234

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briefly: We find statistical rotational distributions and non-statistical spin-orbit branching ratios for the nascent NO al all excess energies up to z 2500 cm- ‘. Minor deviations from statistical product state distributions will, however, not become apparent in our present experiment due to the low sensitivity of our probe technique. The statistical rotational product state distributions indicate that the energy is randomised prior to the separation of the fragments. As with CF,NO, mode specific effects are not apparent in the NO product state distributions, and the dissociation rates will be controlled by the rates of radiationless transition. The nature of the potential energy surface controlling the final dissociation will be established by fluorescence lifetime measurements. For HNO Dixon et al. have established that higher levels of the A(n,x*) state are strongly affected by Renner-Teller coupling to degenerate levels of the ground state which in turn are influenced by spin-orbit coupling to the lowest triplet state T1,E3(n,x*) [18,19]. The higher level density in CCIFzNO and the fact that its A and 2 states are components of a Renner-Teller pair as in HNO is bound to lead to efficient internal conversion to So. However, the S, levels excited at 646.6 nm are substantially above the 13500 cm-’ threshold for dissociation on So. We anticipate that intersystem crossing (ISC) to TI provides the dominant nonradiative transition, and that dissociation proceeds via the triplet surface. Further experiments clarifying the role of the non-radiative processes are necessary if the predissociation mechanism of CCIFzNO A( n,x*) is to be understood in detail. The photofragment yield spectrum (fig. 2) can be readily understood in conjunction with the excitation spectrum [ 8 1. Four members of an anharmonic vibrational progression with a highest (O-l ) interval of 105 cm- ’ are built on the 0: origin which are easily assigned to the torsional vibration of the upper state. A nearly identical torsional progression is associated with one quantum of an upper state rocking mode of 227 cm-‘. A few members of a third torsional progression associated with one quantum of a 343 cm- ’ upper state skeletal bending mode can be followed into the 675 nm region where the fluorescence yield drops drastically, and the NO yield spectrum becomes too congested for an unambiguous

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assignment. Clearly the major geometry changes accompanying this electronic transition are in the dihedral and bending angle of the CNO group as is the case with CF3N0. In CClF2N0, however, dihedral isomerism associated with the torsional potential functions leads to two conformers in the ground and electronically excited states. A detailed analysis [ 17 ] indicates that the two equivalent “gauche” conformations of the ground state with eclipsed F and 0 atoms are more stable than the “cis” conformer with eclipsed Cl and 0 atoms. The very weak satellite transitions noted previously in the excitation spectrum [ 81 and identified by us as “hot” bands originate from the tiny fraction of molecules that get trapped in the energetically higher “cis” well during the course of the expansion. Collision-induced conformational changes in supersonic jet expansions are probably far more common than indicated by the paucity of such observations in the literature. The value of 13500 cm-’ for the C-N bond dissociation energy from the present work compares well with values in CFJNO [ 31 and 2-methyl-2-nitrosopropane [ 71. The accuracy of this result is limited by the poor sensitivity of the two-photon LIF technique. Low dissociation energies for the CN single bond appear to be characteristic of C-nitroso alkanes [20,21]. The present work, illustrates again the advantages of synchronised and electronically delayed pump -probe experiments for both dynamical and spectroscopic studies of predissociating jet-cooled molecules.

Acknowledgement We are grateful to the SERC and its laser Support Facility for a grant and the loan of equipment. We also thank the Carnegie Trust for the Universities of Scotland for a scholarship (to MRSMcC), the SERC for an earmarked studentship (to JAD), and Profes-

8 May 1987

sor H. Reisler and Professor C. Wittig for communicating their results prior to publication.

References [ 11 J.P. Simons, J. Phys. Chem. 88 (1984) 1287. [2] R. Vasudev, R.N. Zare and R.N. Dixon, J. Chem. Phys. 80 (1984) 4863. [ 31 R.D. Bower, R.W. Jones and P.L. Houston, J. Chem. Phys. 79 (1983) 2799. [ 41 J. Pfab, J. Hager and W. I ieger, J. Chem. Phys. 78 ( 1983) 266. [ 51 I. Nadler, J. Pfab, H. Reisler and C. Wittig, J. Chem. Phys. 81 (1984) 653. [ 61 I. Nadler, M. Noble, H. Reisler and C. Wittig, J. Chem. Phys. 82 (1985) 2608. [ 71 M. Noble, C.X.W. Qian, H. Reisler and C. Wittig, J. Chem. Phys., to be published. [ 81 N.P. Emsting, J. Chem. Phys. 80 (1984) 3042. [ 91 N.P. Emsting and J. Pfab, Spectrochim. Acta 36 A (1980) 75. lo] R.G. Bray, R.M. Hochstrasser and J.E. Wessel, Chem. Phys. Letters27 (1974) 167. 1111 M.P. Roellig, P.L. Houston, M. Asscher and Y. Haas, J. Chem. Phys. 73 (1980) 5081. 121 M. Dubs, U. Brtlhlmann and J.R. Huber, J. Chem. Phys. 84 (1986) 3106. I 1310. Benoist d’Azy, F. Lahmani, C. Lardeux and D. Solgadi, Chem. Phys. 94 (1985) 247. 141 C.X.W. Qian, M. Noble, I. Nadler, H. Reisler and C. Wittig, J. Chem. Phys. 83 (1985) 5573. 151 J.B. Halpem, H. Zacharias and R. Wallenstein, J. Mol. Spectry. 79 (1980) 1. [ 161 P.J. Bogan and D.W. Setser, J. Chem. Phys. 64 (1976) 586. [ 171 J.A. Dyet, M.R.S. McCoustra and J. Pfab, to be published. [ 18 ] R.N. Dixon, K.B. Jones, M. Noble and S. Carter, Mol. Phys. 42 (1981) 455. [ 191 R.N. Dixon, M. Noble, C.A. Taylor and M. Delhoume, Faraday Discussions Chem. Sot. 7 1 (198 1) 125. [20] P.J. Carmichael, B.G. Gowenlock and C.A.F. Johnson, Intern: J. Chem. Kinetics 4 (1972) 339. [ 2 1 ] L. Batt and G.N. Robinson, in: The chemistry of amino, nitroso and nitro compounds and their derivatives, ed. S. Patai (Interscience, New York, 1982) p. 1035.

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